The present invention relates to reflectors such as mirrors and reflective polarizers that utilize multilayer interference stacks of various materials to achieve a desired optical performance.
The reader is directed to the glossary at the end of the specification for guidance on the meaning of certain terms used herein.
Thin film birefringent mirrors that comprise birefringent polymer layers are known. See, e.g., U.S. Pat. Nos. 5,808,798 (Weber et al.) and 5,882,774 (Jonza et al.), both of which are incorporated herein by reference, and PCT Publication WO 99/36258 (Weber et al.). Such mirrors can have spectrally broadband or narrowband reflection characteristics as desired by appropriate selection of the individual layer thicknesses and refractive indices. Furthermore, it is known to tailor the out-of-plane (z-direction) refractive indices of two adjacent layers in the optical repeat units of birefringent multilayer mirrors and polarizers so that the reflectivity for the p-polarization component of obliquely incident light decreases slowly with increasing angle of incidence, is independent of the angle of incidence, or increases as the angle of incidence increases. Substantially matching the out-of-plane refractive indices causes the left (short wavelength) bandedges of s- and p-polarized light to be matched, which is useful for certain color beamsplitting applications. Both the left and right (short and long wavelength) bandedges of s- and p-polarized light can be matched over a desired range of angles if the difference between the out-of-plane refractive indices is substantial and of the opposite sign as the in-plane refractive index difference.
To date, reflecting films having these highly desirable angular properties have been fabricated from two or more co-extruded polymeric materials, at least one of which has in-plane refractive indices that change during a post-extrusion stretching procedure. In addition to the unique angular performance capabilities, the polymeric materials and processing techniques used have inherent advantages in certain respects over conventional vacuum deposited thin film reflectors, such as the ability to make very high quality reflectors in high volumes and at relatively low overall cost. However, the processing techniques and/or polymeric materials also have inherent disadvantages in certain other respects, such as difficulty in making low volumes of the film economically, difficulty meeting certain stringent flatness specifications, and difficulty surviving in applications where the film is exposed to substantial amounts of ultraviolet light or to temperatures above about 200 degrees C. High angularity reflective films that avoid one or more of these difficulties would be highly useful in a variety of applications.
Hence, there is a need in the art for precision birefringent reflectors which can be made from inorganic materials and optically flat substrates, using available vacuum coating techniques.
The concept of xe2x80x9cform birefringencexe2x80x9d has long been known in the field of optics, but has been treated largely as a mere curiosity. Unlike conventional birefringent materials, which exhibit different refractive indices as a result of an anisotropic physical structure on a molecular scale, materials that are form birefringent exhibit different refractive indices as a result of an anisotropic physical structure on a scale much larger than molecular but much smaller than the wavelength of light. Such form birefringent materials can be fabricated using conventional vacuum deposition equipment and with conventional inorganic materials that form isotropic layers in most modern optical thin film coatings. Form birefringent films have been demonstrated both as uniaxially birefringent films, in which the in-plane refractive indices nx, ny are substantially equal but different from the out-of-plane refractive index nz, and as biaxially birefringent films in which none of the indices are substantially equal. (Throughout this specification, for convenience, films and their constituent layers and microlayers 30 are considered to lie in the x-y plane of a Cartesian x-y-z coordinate system, even though such films or layers can be flexed or bent, or deposited onto nonplanar substrate.) Both positive and negative uniaxial form birefringent films are known in the art. A negative uniaxial form birefringent film (nx≈ny greater than nz) is described in U.S. Pat. No. 5,196,953 (Yeh et al.) for use as a compensator plate in a liquid crystal display. Positive uniaxial form birefringent films (nx≈ny greater than nz), which contain microscopic columnar structures oriented parallel to the z-axis as a consequence of deposition conditions, are also known. See for example xe2x80x9cEffective Principal Refractive Indices and Column Angles for Periodic Stacks of Thin Birefringent Filmsxe2x80x9d, J. Opt. Soc. Am. A, Vol. 10, No. 9, September 1993, pp. 2065-2071, or xe2x80x9cDeposition, Characterization, and Simulation of Thin Films With Form Birefringencexe2x80x9d, SPIE Advances in Optical Materials (1984), Vol. 505, pp. 228-235. Biaxial form birefringent films are discussed in U.S. Pat. No. 5,638,197 (Gunning, III et al.), incorporated herein by reference, as compensation devices in liquid crystal displays.
However, the benefits of using such form birefringent materials in high angularity reflective films, where the z-index of adjacent layers is tailored to achieve a desired angular behavior in the reflection or transmission of a polarizer or mirror, have not been taught or appreciated by others. Such benefits are particularly important for polarizing beamsplitters and color separation filters required to work at high angles of incidence, most notably where the reflector is immersed in a high index medium such as glass. Such applications of both mirrors and reflecting polarizers require the careful control of the reflectivity of p-polarized light compared to that for s-polarized light at oblique angles, which in turn requires careful control of the z-index.
Disclosed herein are reflectors such as mirrors or reflective polarizers that comprise a plurality of thin film optical repeat units to achieve reflection or transmission of light as a function of wavelength, polarization state, and direction of incidence. The reflectors include in a plurality of the optical repeat units at least one optical layer that is form birefringent. Further, the form birefringent layer and another optical layer in the optical repeat units have z-indices that are tailored to produce a desired optical effect as a function of incidence angle. Sometimes, this corresponds to z-indices for such layers that differ by no more than about 80%, more preferably by no more than about half, and even more preferably by no more than about 20%, of the maximum in-plane mismatch between such layers. In some cases, however, the desired z-index difference is large and of opposite sign as the in-plane index differences, or in the case of biaxial birefringent polarizers, as large as possible and of opposite sign as the largest in-plane refractive index difference, but not larger in magnitude than such largest in-plane index difference.
In some embodiments, the optical repeat units include a negative uniaxial form birefringent optical layer. In other embodiments, the optical repeat units include a positive uniaxial form birefringent optical layer. In some of the embodiments both negative uniaxial and positive uniaxial form birefringent optical layers are included. In still other embodiments, biaxial form birefringent layers are included.